Advancement in the integrated circuit technology has led to vast improvements in the speed of integrated circuits. Increasing integrated circuit speeds has resulted in faster rise and fall times of output voltages in response to new inputs generating more system switching noise. Controlling output edge rates and impedances is required to minimize this noise when transmitting data at high rates. The output characteristics are a function of a number of factors, one important factor being the actual load presented to an output terminal. Other factors that affect output characteristics are temperature variations, process variations and voltage variations.
Such things as temperature variations, process variations and voltage variations are internal parameters that can be compensated. One technique for accounting for some of these variations can be found in U.S. Pat. No. 5,220,208, which is incorporated herein by reference. The '208 patent discloses a system that utilizes a time reference and a delay string through which a signal is propagated. The time reference allows accurate measurement of the propagation distance of the signal through the delay string and thus, provides a benchmark by which to judge internal variations. These can then be accounted for. However, this type of system does not provide any information as to the variations associated with different output loads. Some systems that have addressed the problems of variations of the output characteristics as a function of load have utilized constant current sources and some type of analog circuitry to solve this problem. However, these type of systems typically require a higher power drain due to the use of the constant current sources and analog circuitry fabricated on the integrated circuit. With most CMOS systems, this is difficult to implement and may require the use of BiCMOS technology in order to realize these functions.
Therefore, it may be seen that a need has arisen for a method and apparatus for controlling the output as a function of load such that it can be maintained within acceptable limits independent of load.